High strength steel containing chromium and sulfur

ABSTRACT

AN IMPROVED STEEL CONTAINING 0.05 TO 0.40 PERCENT SULFUR WHICH HAS A HARDNESS GREATER THAN 50 ROCKWELL C, GOOD MACHINABILITY AND HIGH FATIGUE STRENGTH AND IS PARTICULARLY SUITED FOR BEARINGS, GEARS, ETC. WHICH ARE SUBJECT TO REPEATED STRESSES.

March 2, 1971 J. MQNNOT 3,567,435

HIGH STRENGTH STEEL CONTAINING CHROMIUM AND SULFUR Filed Nov. 24. 1967 v(1| '|o* loo CYCLES. (10) INVENTOR. Jacques Monnot BY w hk fim qokl-smk HIS ATTORNEYS United States Patent 01 see 3,567,435 Patented Mar. 2, 1971 Int. (:1. C22c 39/14 US. Cl. 75-126 4 Claims ABSTRACT OF THE DISCLOSURE An improved steel containing 0.05 to 0.40 percent sulfur which has a hardness greater than 50 Rockwell C, good machinability and high fatigue strength and is particularly suited for bearings, gears, etc. which are subject to repeated stresses.

This invention relates to an improved high strength steel; more particularly it relates to a hard steel containing 0.05 to 0.40 percent sulfur which has an improved fatigue strength and endurance to repeated stresses.

It is well known that intentional alloying additions of sulfur to steels result in a product having improved machinability. Such additions may increase the sulfur level to .1 percent or more. The sulfur forms a sulfide, an oxysulfide, or a combination thereof with iron or with other elements that have been used as alloying additions in the steel. It is believed the various sulfide inclusions act as a lubricating agent for the working tool, drastically reducing the normal friction between the working tool and the steel workpiece, and thereby improving the machinability of the steel. However, such sulfur additions have been used only in steels of low or medium hardness, at least below 50 Rockwell C. In such cases the mechanical properties of the steel have been considered adequate for certain purposes.

It is also well known that nonmetallic inclusions have deleterious effects on the mechanical properties of the product. The nonmetallic inclusions serve as stress risers and often result in incipient cracking initiated from the area in the steel adjacent the inclusion. This effect is especially apparent where the steel product is exposed to repeated stresses. And, as the hardness of the product increases, the deleterious effect of such inclusions increases. For example, below a hardness of about 32 Rockwell C, the fatigue limit of the treated structural steels is about one half of their conventional ultimate tensile strength. However, the ratio of fatigue limit to ultimate tensile strength decreases rapidly as the hardness of the product increases. Many steels are subjected to vacuum degassing and the like in order to appreciably decrease the amount of nonmetallic inclusions.

Specifications for steels in many countries limit the number, the size, and the distribution of nonmetallic inclusions, and particularly sulfide inclusions, for various uses. Such specifications apply to steels which are to receive hardening treatments, either surface treatment or mass treatment, and particularly those steels which are to attain hardnesses of above 50 Rockwell C. This applies whether the hardness treatment be by chemical or thermal methods. Minimum inclusion specifications also apply to high strength steels having high mechanical properties such as steels to be used for ball or roller bearings, structural members, gears, aircraft applicatons, and the like. In general the limiting specifications apply to all hard steels when the product to be made therefrom will be subjected to repeated stresses.

For the above reasons, it is not surprising that in the manufacture of high strength steels, which are subjected to repeated stresses, it has been considered inadvisable to add materials which will create nonmetallic inclusions and particularly sulfur to the steel. Therefore, the sulfur content in such steels has been maintained at its residual level; namely, in amounts up to 0.035 percent. However, we have found that deliberate additions of sulfur to bring the sulfur content to between 0.050 and 0.400 percent, and preferably between 0.050 and 0.150 percent, not only improves the machining properties of the steel bult also greatly improves the mechanical properties of the steel.

In the single figure drawing we have shown compositive results of fatigue tests on a steel within the scope of my invention and a steel of the prior art.

My invention relates to high strength steels for use as bearings, engine parts and structural members in which improved fatigue strength and endurance to repeated stresses are essential. More particularly, my invention relates to steels having a hardness greater than 50 Rockwell C, a hardness which may be obtained by thermal or chemical treatment either in the mass or locally. The steels of this invention are characterized by having a sulfur content of between 0.05 to 0.40 percent by weight. I prefer that the sulfur content be maintained between 0.05 and 0.150 percent by weight.

The source of the sulfur can be any of the commercially available sources known in the art, such as iron pyrites and the like or free sulfur itself. The sulfur is generally added after the refining operation has been completed. The refining procedures well known to those skilled in the art for machining steels may be utilized. The sulfur may be added before or after the other alloying additions although it is preferable to add the sulfur after other additions are made.

I have found that the addition of sulfur to the high hardness steels not only results in the obvious improved machining characteristics, but contrary to the general opinion, certain mechanical properties are also enhanced. Specifically, it has been found that the fatigue limit has been improved when the sulfur content is higher than 0.05 percent, with this improvement being maintained up to sulfur contents of about 0.4 percent.

The general analysis of bearing steel of my invention in percent by weight is as follows:

To demonstrate the improved qualities of the steels of my invention, two 2-ton ingots having the analysis shown in Table I were prepared and tested under identical conditons. Ingot A had a compositio of normal bearing steels while Ingot B had a composition of my novel steel. In Ingot B the additional sulfur content was obtained by the addition of iron sulfide after refining and deoxidation.

TABLE I Ingot A, Ingot B, percent percent 1 1 1. 5 1. 5 Sulfur 008 102 Iron and impurities. B alance Balance Rotary bending fatigue tests in the longitudinal direction were performed on Moore test specimens on samples from both ingots. The test specimens had previously been treated by oil-hardening after a twenty minute soak in a salt bath at 850 C. followed by tempering at 170 C. for one hour. This heat treatment resulted in a hardness of 62 Rockwell C. The results of these tests are shown by the corresponding Wohler curves of the drawing. It should, 5

be noted that the fatigue limit for 10 cycles is appreciably higher for the samples from Ingot B. This improved fa tigue limit is greater than 6 H bar at the various cycles; Comparable improvements have been'observed in transverse test specimens as well as longitudinal test specimens. i i T While I have described my invention in terms of the present preferred embodiments, it should be nnderstood that it may be otherwise embodied within the scope of the following claims.

' I claim:

1. Bearings made from bearing steels consisting essentially of 0.90 to 1.15 percent carbon; up to 1.5 percent manganese; 0.5 to 2.0 percent chromium; up to 0.3 percent molybdenum; up to 0.3 percent vanadium; up to 1.0 percent silicon; 0.05 to 0.40 percent sulfur; and the balance iron and incidental impurities.

2. Bearing steels having high strength and fatigue resistance and a hardness greater than 50 Rockwell C, said bearing steels consisting essentially of 0.90 to 1.15% C; up to 1.5% Mn, 0.5to 2.0% Cr; up to 0.3% Mo; up to 0.3% Va; up to 1.0% Si; 0.05 to 0.40% S, and the balance iron and incidental impurities.

3. The bearing steels of claim 2 wherein the sulphur content is between 0.05 and 0.15% by weight.

fl. The bearings of claim 1 wherein the sulfur content is between 0.05 and 0.15% by weight. 7

References Cited UNITED STATES PATENTS 1 1,252,596 1/1918 McMillin 75-1 26L 1,831,946 11/1931 Breeler 75126L 2,793,947 5/1957 Swanson 75 126 3,144,362 8/1964 Bradd 75126 3,060,016 10/1962 Melloy L 75126 HYLAND BIZOT, Primary Examiner UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 567 435 Dated March 2 1971 Jacques Monnot Inventor(s) It is certified that error appears in the above-identified pater and that said Letters Patent are hereby corrected as shown below:

Column 1 line 65 after "steels" insert or steels Signed and sealed this 15th day of June 1971 (SEAL) Attest:

EDWARD M.FLETCHER,JR. WILLIAM E. SCHUYLER, Attesting Officer Commissioner of Pate 

